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5. Single Number

easyAsked at Rappi

Find the one element that appears once when every other element appears twice — Rappi frames this as finding the odd-out courier ID in a paired-shift roster.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a non-empty integer array where every element appears twice except one, find the single one. Your algorithm should run in linear time and use constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Each element except one appears twice

Examples

Example 1

Input
nums = [2,2,1]
Output
1

Example 2

Input
nums = [4,1,2,1,2]
Output
4

Approaches

1. Hash map count

Count occurrences and return the key with count 1.

Time
O(n)
Space
O(n)
const cnt = new Map();
for (const n of nums) cnt.set(n, (cnt.get(n)||0)+1);
for (const [k,v] of cnt) if (v === 1) return k;

Tradeoff:

2. XOR accumulator

XOR cancels duplicate pairs to zero, leaving the unique value — constant space, one pass.

Time
O(n)
Space
O(1)
function singleNumber(nums) {
  let x = 0;
  for (const n of nums) x ^= n;
  return x;
}

Tradeoff:

Rappi-specific tips

Rappi loves the XOR trick because it mirrors how their dispatch ledger reconciles paired pickup/drop-off events in constant memory.

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Output

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